Metastatic cancer largely remains an incurable disease that requires development of novel therapeutic strategies. While the accumulating knowledge on molecular basis of cancer offers new potential targets for anticancer drugs, there is an alternative approach that relies on mechanisms developed through the millions of years of human co-existence with viruses. The viruses emerge as promising instruments against cancer. Substantial selectivity of infection and replication in cancer cells is characteristic of many viruses, and their therapeutic efficiency and safety can be improved by genetic manipulations, or by directed bioselection.
The idea of using viruses in the treatment of malignancies dates back to the beginning of the 20th century when reports on cases of spontaneous tumor regression after viral diseases or vaccination started to appear [1-6]. However, it took several decades of intense studies of the complex relations between viruses and their hosts until viruses started to be considered as potential tools for cancer therapy [7]. Modern studies on oncolytic viruses represent a dynamic and exciting field that absorbs the most recent discoveries in molecular, cell and cancer biology. Viruses can be quickly modified by recombinant DNA technology thereby rapidly incorporating the fast growing knowledge into oncolytic virus design. The studies involve a wide array of viral species belonging to diverse viral families, such as adenoviruses, herpesviruses, parvoviruses, enteroviruses, reoviruses, rhabdoviruses, paramyxo- and myxoviruses, alphaviruses, myxoma viruses and poxviruses. Examples of large randomized trials include phase III trial of an attenuated strain of herpes simplex virus-1 (HSV-1) that encode human granylocyte-macrophage colony-stimulating factor (GM-CSF) in the treatment of patients with metastatic melanoma [8]; phase II trial of reovirus in combination with chemotherapy in patients with head and neck cancer [9] and phase II trial of genetically engineered oncolytic poxvirus JX-594 in patients with hepatocellular carcinoma [10]. These trials confirm that oncolytic viruses do not produce substantial side effects and have a considerable antitumor efficacy affecting the overall patient survival.
Many members of the paramyxovirus family are now being considered as potential promising tools in cancer therapy. Among them Newcastle disease virus, Measle virus, Mumps Virus and Sendai Virus [11-13].
Sendai Virus
Studies of the anticancer effects of Sendai virus (SeV), which is also known as murine parainfluenza virus type 1 or hemagglutinating virus of Japan, are proceeding in several countries. The SeV virus belonging to the Respirovirusgenus is responsible for a highly transmissible respiratory tract infection in mice, hamsters, guinea pigs and rats [14]. It can be detected in mouse colonies worldwide. While SeV spreads in rodents through both air and direct contact routes [14], it is considered very safe for humans. One strain of SeV was approved in USA for a small human clinical trial for the prevention of a pediatric disease caused by parainfluenza type 1 virus. The trial demonstrated that the intranasal administration of SeV was well-tolerated and protecting against human parainfluenza virus infection[15].
Sendai virus has oncolytic properties. Genetically engineered recombinant Sendai virus (rSeV) disseminates extensively in human tumor cells xenotrasplanted to nude mice without spreading to the surrounding normal cells [16]. This dissemination leads to the inhibition of tumor growth in the mice. The tested tumor cells include fibrosarcoma, pancreatic epithelioid carcinoma and human colon carcinoma [16]. A significant reduction of tumor growth, including the complete elimination of established brain tumors, was demonstrated in murine models in a study using a different rSeV strain [17]. Similar results were obtained with mouse xenografts of human sarcoma and prostate cancer [16, 18]. Recombinant SeV efficiently eliminated tumors in rat models, including melanoma, hepatocellular carcinoma, neuroblastoma, squamous cell carcinoma and prostatic cancer [19].
Remarkably, the replication of SeV is not absolutely required for the oncolytic effects of SeV as even UV inactivated virus was shown to be efficient against colon[20, 21] renal[21] and prostate carcinomas in mouse models [22].
The UV inactivated virus with enhanced antitumor activity was constructed by conjugation of interleukin-12 with hemagglutinin-neuraminidase (HN)-depleted viral particles (HVJ-E). It was demonstrated that this novel immune-stimulatory pseudovirion suppresses lung metastatic melanoma growth by regionally enhancing IFN-gamma production without increasing the serum IFN-gamma level [23].
In all mentioned studies SeV eradicated the tumors or significantly inhibited their growth. No regular clinical studies with SeV have been carried so far, although the apparent safety of the virus and the promising preclinical data suggest that SeV could be an excellent candidate for oncolytic virotherapy. Supporting these hopes is a case of a short-term remission in a patient with acute leukemia following an intravenous injection of live SeV described back in 1964 [24].
Mechanisms of Viral Antitumor Activity
Oncolysis Mediated by Direct Killing During Selective Replication of Paramyxoviruses in Cancer Cells
The cancer cells are generally better hosts for viruses than normal cells. Cancer cells usually acquire defects in the protective mechanisms that resist viral replication. This also relates to paramyxoviruses. The upregulation of p53 by IFNs plays protective role against the emerging cancer cells. Therefore, cancer cells experience strong selection pressure against both the p53 and the IFN-mediated mechanisms. During the malignant progression cancer cells acquire mutations in different components of the IFN system, p53 and apoptotic pathways that allow them to escape from the host regulation and to expand. But the very same defects that promote tumor growth provide the opportunity to destroy cancer cells with the use of oncolytic viruses.
Dissection of the general and specific mechanisms of complex interactions in viral oncolysis is important for designing new variants of oncolytic paramyxoviruses that would direct their potentials toward a more safe and efficient elimination of cancer cells through the direct killing and the stimulation of anticancer immunity. Studies on viral oncolysis mediated by paramyxoviruses have revealed several independent mechanisms that collectively contribute to their strong anticancer effects.
Role of Broken Innate Antiviral Defense Mechanisms in Cancer Cells
While pretreatment with the endogenous or exogenous IFN-beta completely inhibited replication of Newcastle Disease Virus (NDV) (a representative of Paramyxoviruses) in normal cells, no effect on viral replication was observed in tumor cells [25]. Similarly, while normal macrophages with intact IFN system were not susceptible to NDV infection, the macrophage-derived tumor cell lines showing low responsiveness to IFN-alpha and low expression level of IFN-beta were permissive for replication of the virus [26].
Cancer cells show defects in the IFN induction in response to NDV infection. Infection of several tumorigenic human cell lines with NDV showed a delayed response in the expression of antiviral genes such as PKR and MxA. While in non-tumorigenic peripheral mononuclear blood cells (PMBC) the replication cycle of NDV is stopped after the production of positive-strand RNA, it keeps going in tumor cells [27]. However, while the deficiency in IFN pathways is quite common in cancer, it cannot be regarded as a requirement for the increased sensitivity to viruses: some cancer cell lines show high sensitivity to killing by NDV even though they retain the responsiveness to IFN treatment [28, 29].
The replication of NDV in cancer cells results in death of infected cells. Efficient apoptosis was observed in a study involving 14 human cancer cell lines of ecto-, endo-, and mesodermal origin [28]. Although cancer cells infected with NDV eventually die, the apoptotic pathways are not necessarily fully intact in cancer cells. In fact, the increased sensitivity of cancer cells to virus infection is enforced by the attenuated apoptosis; it gives the cells enough time for successful viral replication and consequent viral spread. In a non-small-cell lung carcinoma cell line A549 that is relatively resistant to apoptosis due to overexpression of the anti-apoptotic protein Bcl-XL the oncolytic activity of NDV was significantly enhanced, even though the cells were capable of a robust type I IFN response[30]. In another study the overexpression of anti-apoptotic protein IAP Livin correlated with the enhanced oncolytic activity of NDV chemotherapy-resistant melanoma cells. These melanoma cells also demonstrate an intact IFN response [31]. The examples indicate that the increased sensitivity of cancer cells to viral infection can be achieved through different mechanisms that include defective IFN system, or compromised apoptotic response.
Formation of Syncytium Contributes to Viral Oncolysis
Some paramyxoviruses can achieve spreading of virus infection independent on the release of mature virus particles. The mechanism includes fusion of infected cells with their non-infected neighbors. The fusion results in the formation of a syncytium, a large polykaryonic conglomerate originating from many cells. For a single infected cell up to 50-100 neighboring cells may fuse together to form a syncytium[32]. The process of cell fusion allows the virus to avoid the likely exposure to host neutralizing antibodies. Usually the syncytia survive no longer than 4-5 days and it is unclear to which extent the apoptosis plays a role in their death. Experiments with caspase inhibitors suggest that apoptosis is not the only mechanism responsible for the fusion induced cell death [33]. It was proposed that the syncytia die through a processes that, by multiple morphological and biochemical criteria, bear very little resemblance to the classical apoptosis [34, 35].
It is likely that the ability of some viruses to induce the formation of syncytia may correlate with their oncolytic potential. The fusion protein of NDV was introduced into an oncolytic strain of vesicular stomatitis virus (VSV), which significantly enhanced oncolytic activity of the virus against multifocal hepatocellular carcinoma in the livers of immunocompetent rats [36]. Similarly, the oncolytic activity of a strain of replication-competent herpes simplex virus (HSV) was significantly increased after the introduction of a hyper-fusogenic glycoprotein from the gibbon ape leukemia virus [37]. The fusogenic potential could be further increased by the introduction of amino acid substitutions. A novel NDV variant harboring an L289A substitution within the F gene possesses an enhanced fusion and oncolytic activities against rat hepatocellular carcinoma cells, both in vitro and in immunocompetent rats [38]. A mutant of SV5 paramyxovirus that harbors a glycine-to-alanine substitution in the fusion protein was hyper-fusogenic and displayed an enhanced oncolytic activity [39]. Remarkably, even plasmid vectors [30, 40, 41] or replication-deficient viruses [42-44] expressing the fusogenic membrane glycoproteins are capable of promoting significant tumor regressions, suggesting that the cell fusion may substantially contribute to the oncolytic activity of paramyxoviruses.
Oncolysis Mediated by the Virus-Induced Stimulation of the Antitumor Immune Response
In addition to the direct killing of infected cancer cells viral infection elicits systemic responses that contribute to viral oncolysis.
Many viral and bacterial pathogens have a tendency to accumulate in the primary tumors and metastases, so viral particles concentration increases in tumor tissue. In short, such process can be called tumor xenogenisation. This term “xenogenisation” is a neologism. Pathogens bring into tumor mass a lot of biological non-self-material. This alien material is rich with foreign antigens that have immuno-triggering properties. The process can be called by new term derived from Greek “xeno” (meaning alien, stranger).
Appearance of foreign virus proteins in the infected cancer cells increases visibility of these cells by immune system.
The systemic response to viral infection includes stimulation of mechanisms of antiviral innate immunity, such as the production of IFNs, other cytokines and the activation of natural killers and T-lymphocytes. Also, viral infection elicits adaptive immune responses that not only act to contain the infection by targeting the released viral particles and the infected cells, but also assists in the exposure of cancer cells to the antigen-presenting cells leading to activation of the anticancer immune response.
Most tumors are MHC class I positive, but negative for MHC class II. MHC class I molecules are one of two primary classes of major histocompatibility complex (MHC) molecules, they are found on nearly every nucleated cell of the body. MHC class II molecules are secondary in two primary classes of major histocompatibility complex (MHC) molecules. They are expressed in professional, immune antigen presenting cells, such as dendritic cells but may also be induced on other cells including malignant by INF-gamma.
SeV is capable of INF-gamma stimulation that activates immunoproteasomes and triggers MHC class II production in cells, which are not in professional, immune antigen presenting cells, including malignant cells. IFN-gamma stimulates the expression of class I MHC molecules as well in many cells including malignant and it triggers production of co-stimulatory molecules on antigen presenting cells. This leads to extra antigen presentation of peptides from mutated-abnormal proteins produced in malignant cells. Consequently, malignant cells became more visible for immune system. IFN-gamma promotes the differentiation of naive helper T cells; it activates cytotoxic T cells and increases the cytotoxicity of NK cells. NK cells contain small granules in their cytoplasm with proteins such as perforin and proteases. Upon release in close proximity to a malignant cell, perforin forms pores in the cell membrane of the target cell, inducing either apoptosis or osmotic cell lysis. In addition IFNγ activates macrophages for phagocytosis and lysis.
Virus induced IFNs and cytokines stimulate dendritic cells (DCs) that educate cytotoxic T-lymphocytes to target tumor cells. When exposed to target malignant cells, cytotoxic T-lymphocytes release the cytotoxins perforin, granzymes, and granulysin that lead to cancer cell death. In addition viruses inside tumor mass might be covered by antibodies that in turn also could be detected by immune cells. Antibodies that bind to antigens can be recognized by receptors expressed on NK cells resulting in NK activation, release of cytolytic granules and consequent cancer cell destruction.
Some extra immune stimulation can occur because of host reaction to viral RNA even without viral infection, but after contact between host cell and non-pathogenic virus. There are immune system mechanisms that are sensitive to double stranded RNA (dsRNA). When the dsRNA is exogenous (coming from a virus particle with an RNA genome), the RNA is imported directly into the cytoplasm and cleaved to short fragments by the enzyme. RNA molecules shorter than about 25 nt largely evade detection by the innate immune system, which is triggered by longer RNA molecules. Most cells of the body express proteins related to the innate immune system, and upon exposure to longer than 25 n. RNA double stranded fragments of exogenous origin, these proteins initiate signaling cascades that result in inflammation. This inflammation might hypersensitize the malignant cells to immune system attack.
In theory, to some degree, anticancer immune activation can occur even without viral accumulation in tumor mass. Viral particles even being injected into skin and even being inactivated by antibodies can trigger some of the events described above.
Paramyxoviruses Stimulate the IFN Response
There is a considerable overlap in functions of innate and adaptive immune systems that protect against viral infection and against cancer. IFNs are glycoproteins secreted by cells in response to viral infection and helping neighboring cells to resist viral invasion. However, IFNs also have important functions in immunosurveillance and elimination of cancer cells [45, 46]. The released IFNs bind to their respective receptors and trigger signaling pathways that modulate broad biological responses including antiviral, growth inhibiting, immunomodulatory and pro-apoptotic [47, 48]. Among these responses IFNs limit viral spread by increasing the p53 activity and promoting apoptosis of the virus-infected cells.
Paramyxoviruses are known to be strong inducers of IFNs, the property that has been used in biotechnology: the Sendai Virus was chosen among many other viruses for industrial-scale production of IFN from human leukocytes [49]. In Human Peripheral Blood Leucocytes (HPBL) SeV behaves as a potent inducer of IFN-alpha [50], it induces the secretion of at least nine different IFN-alpha species [51]. In HPBL SeV also stimulates the production of IFN-gamma [52].Similarly, the NDV stimulates the production of IFN-alpha from several gene isoforms [53]. The induction of IFN can initiated following the recognition of dsRNA (particularly those containing 5′-triphosphates), which is synthesized during viral replication. The dsRNA is recognized by two types of pattern-recognition receptors, an endosomal TLR3 [54] and a cytoplasmic helicase RIG-1 [55, 56], which trigger the activation of transcription factor IRF3 controlling the expression of IFN genes. However, in HPBL the IFN secretion can be also induced by the FIN-protein a process independent of the dsRNA response [53, 57]. Apparently, the two alternative mechanisms of IFN induction explain the high interferon stimulating capacity of paramyxoviruses [57, 58].
IFNs exert complex systemic effects that make them useful means for adjuvant therapy of cancer [59, 60]. IFNs have been used in therapeutic schemes for the treatment of metastatic melanoma, renal carcinoma, Kaposi sarcoma, bladder carcinoma hairy-cell leukemia etc. The significant prolongation of a disease-free survival has now been largely validated by the combined analyses of multi-institutional trials and by the subsequent studies that have included meta-analyses [47]. IFNs exhibit strong antiproliferative and pro-apoptotic activities that might inhibit expansion of cancer cells and explain some of the therapeutic effects. INFs can act systemically by regulating the immune response through the activation of dendritic cells, cytotoxic T cells, and natural killer cells. IFNs markedly increase the MHC class I and class II-dependent antigen presentation that can cause increased presentation of tumor specific antigens by malignant and professional antigen presenting cells. This, in turn, can stimulate proliferation and antitumor activity of cytotoxic T lymphocytes.
The responses to IFNs are dysfunctional in many malignancies making the direct effects of IFNs less efficient. However, IFNs can affect tumor growth indirectly, through suppression of angiogenesis by altering the stimuli from tumor cells and by inhibiting endothelial cells. The degree of inhibition correlates with the reduced tumor vascularization and the consequent retardation of tumor growth [45, 47, 61]. The treatment with IFNs may also affect the viability of cancer stem cells that are highly resistant to chemo- and radiotherapy and are responsible for the disease relapses. Most likely, IFNs affect tumor vasculature and disrupt the vascular niche of stem cells, as it was found in mouse xenografts of human glioma [60].
Paramyxoviruses Stimulate Secretion of Cytokines
In HPBL SeV was shown to stimulate the synthesis of interleukins (IL) 2, 6, 8, macrophage inflammatory protein-1 alpha (MIP-1-alpha), MIP-1-beta, RANTES, and MCP-1[50, 52]. Live or UV-irradiated SeV can stimulate the IL-6 release in the treated animals [20]. In dendritic cells the incubation with SeV results in the IL-6 secretion; the fusion protein (F) was found to be responsible for the effect [62]. The injection of the UV-inactivated SeV into established mouse renal cell carcinoma tumors resulted in the secretion of chemokine CXCL10 by infiltrating dendritic cells [21]. In a number of tested human tumor cells NDV was also shown to be an inducer of the CXCL10 secretion [63]. The CXCL10 was found to have an antitumor activity through the attraction of monocytes, macrophages, T cells, NK cells, and dendritic cells, the promotion of adhesion of T cells to endothelial cells, and the inhibition of angiogenesis [64]. In HPBL incubation with NDV also stimulates the secretion of cytokines TNF-alpha [65, 66] and TRAIL [53, 66]. It appears that the induction of TRAIL is due to a single viral protein FIN [53, 67]. In monocytes the NDV was also shown to induce the secretion of TRAIL, and to kill various human tumor cell lines following the stimulation of TRAIL-R2 receptors [67]. Macrophages could be also stimulated by incubation with NDV leading to up-regulation of a set of macrophage-specific genes and to secretion of TNF-alpha [68].
Stimulation of Natural Killer Cells by Paramyxoviruses
Natural killer (NK) cells constitute a type of cytotoxic lymphocytes that bridge the innate and adaptive branches of the immune system [69]. The NK cells participate in the early control against virus infection and in tumor immune surveillance. These cells have the ability to recognize the abnormal or stressed cells in the absence of antigen-specific cell surface receptors and MHC and destroy them, allowing a much faster protective effect. They do not require activation in order to kill cells that are not expressing “self” markers of MHC class I. Many viruses induce down-regulation of the MHC Class I molecules on the cell surface making the infected cells preferable targets for NK cells [70]. The NK cells widely interact with other components of the immune system affecting subsequent antigen-specific T and B cell responses [69].
The NK cells display natural cytotoxicity receptors (NCRs) responsible for their activation. Among these receptors are NKp46 and NKp44. It was found that the HN protein of paramyxoviruses directly interacts with the NCRs NKp44 and NKp46 and triggers the NK-mediated cells lysis [71-74]. Studies with the UV-inactivated SeV virus highlight the important role of NK cells in the virus mediated oncolysis. In a mouse renal cancer model the UV-inactivated SeV produced strong anticancer effect, which was compromised by co-injection of an NK-depleting anti-GM1 antibody [21].
Stimulation of Dendritic Cells
Dendritic cells (DCs) are professional antigen-presenting cells that can efficiently stimulate innate as well as adaptive immune responses against various pathogens and cancer cells. After sensing a virus or other pathogens, the DCs enter a maturation program and became competent for education of naïve T cells.
The UV inactivated SeV can cause extensive tumor infiltration by dendritic cells (DCs) [75]. The ex vivo infection of DCs by recombinant SeV can induce DCs maturation and activation in only one hour [76]. Treatment with activated DCs harboring different variants of recombinant SeV can improve dramatically the survival of animals inoculated with cells of malignant melanoma, neuroblastoma, hepatocellular, squamous cell, colon and prostate carcinomas [19, 77-80]. The administration of such DCs before tumor inoculation has demonstrated a preventive effect against lung metastasis of neuroblastoma [81] and prostate carcinoma [82]. Another response of DC cells to virus infection is inhibition of immunosuppression mediated by the regulatory T-cells. In experiments with the UV-inactivated SeV it was found that carbohydrates of the viral F-protein are recognized by an unknown receptor(s) on DC cells leading to the NF-kappa B activation and secretion of IL-6 [75].
Role of Tumor Specific Cytotoxic T-Cell Mediated Activity
Infection of tumor cells with NDV can stimulate the tumor-specific cytotoxic CD8+T-cell (CTL) response and increase the CD4+T-helper cells' activity even in the absence of an antiviral T-cell response [83]. The activation and infiltration of tumor sites with CTLs is the result of complex effects of viral infection on the immune system, including local and systemic release of IFNs and other cytokines. Localized inflammation associated with the in situ cytokine production may contribute to the anti-tumor response [84]. Interestingly, the UV-inactivated virus was as active in promoting the tumor-specific CTL response as the infectious NDV. It was found that this effect is mediated by the presence on the tumor cell surface of functional viral 1-1N molecules that have affinity to plasma membranes and therefore can mediate a strong adhesion of the infected cells to CTLs [83]. Since the 1-1N proteins of SeV and NDV are highly homologous, the data suggest that the 1-1N protein, regardless of whether it derives from SeV, NDV or other related paramyxoviruses, can activate both NK and CTL responses.
Role of Viral Sialidase Activity of the HN Protein in Paramyxovirus Oncolysis
Sialic acids represent N- or O-substituted derivatives of neuraminic acid, a monosaccharide with a nine-carbon backbone found in several classes of cell surface and secreted glycan molecules. The sialic acids provide the hydrophilicity to vertebrate cell surfaces, and act as receptors for certain pathogens and toxins. The sialic acids play important structural roles, as they bind selectins, components of intracellular matrix. Metastatic cancer cells often express a high density of sialic acid-rich glycoproteins that increase the invasive potential. The overexpression of sialic acid on the surfaces creates a negative charge on cell membranes, leading to repulsion between cells that promote metastases by helping cancer cells' entry into the blood stream [85, 86]. In a mouse model it was found that the ability of tumor cells to metastasize from subcutaneous sites depends on the abundance of sialic acids [87]. In a model of non-invasive revertants of a highly metastatic mouse T-cell lymphoma the metastatic potential was lost coincident with the loss of sialylation[88]. A similar effect was observed for a Friend Leukemia cell line[89], in models of T-cell hybridoma, [90] melanoma [91] and colon carcinoma [92]. The extent of cell surface poly-sialylation was suggested as a marker characterizing the differentiation status of thyroid and small-cell lung carcinoma cells [93].
A more detailed analysis revealed qualitative differences in sialic acids on the surface of cells displaying different degrees of invasiveness. While in normal human colon cells, as well as in adenomas and several carcinomas of different grades an alpha2,3-linked sialic acid was detected, in the highly malignant variants alpha 2,6-linked sialic acids were also present. It was found that the malignant progression was associated with the de novo expression of an alpha 2,6 sialyl-transferase, which transforms the alpha 2,3-linked sialic acid into the 2,6-linked sialic acid [94]. The increase in alpha 2,6 sialylation coincident with tumor progression was also detected for hepatocellular carcinomas [95] and in colon adenocarcinomas [96-99].
Treatment with inhibitors of the sialylation process was shown to decrease the malignancy of cancer cells. Soyasponin I, a potent 2,3 sialyltransferase inhibitor, can diminish the metastatic and invasive properties of breast cancer [100] and melanoma cells [101]. A derivative of lithocholic acid that is also an inhibitor of sialyl transferase can reduce the metastatic potential of lung carcinoma cells [102].
One of the possible mechanisms linking the increased sialylation with malignant phenotype is the creation of a thick “coat” on the cell surface that hides cancer antigens and provides an escape of malignant cells from the immunosurveillance. For example, it was found that the tumor specific Lewis antigen of malignant medullary thyroid cancer can be masked from the immune system by both alpha 2,3- and alpha 2,6-linked sialic acid residues [103]. Removing some sialic acid residues from the surface of malignant cells by sialidase can unmask cancer specific antigens and make cells visible to the immune system.
The removal of sialic acids from tumor cells was associated with the reduced growth potential. Cytotoxic activity of NK cells was found to be dependent on sialic acids on the surface of tumor cells. Removal of sialic acids by sialidase resulted in the activation of NK cells and secretion of IFN-gamma [104].
The FIN proteins present in SeV, NDV and some other paramyxovirus exposes both erythrocyte agglutinating and neuraminidase (sialidase) activities [105, 106] Neuraminidase cleaves sialic acid on the cell surface [107]. These viruses use sialic acid-containing receptors on the cell surface to penetrate into cells due to the affinity of viral neuraminidase to sialic acid. The abundant presence of sialic acids on the surface of cancer cells can promote the preferential association of the virus with malignant rather than with normal cells and additionally contribute to the selective cytolytic effect in tumors and in the metastases. In particular, the above explanation for tumor selectivity was suggested in a study demonstrating high sensitivity to the SeV mediated lysis of sialic acid-rich prostate carcinoma cell lines PC3 and DU145 in comparison with normal prostate epithelium[22].
It was found that viral neuraminidase can remove sialic acid residues from the surface of malignant cells [106, 108] leading to a dramatic compromise of the ability of B lymphoma cells to induce T-cell activation[108]. The enzyme of NDV can cleave alpha 2,3-, alpha 2,6-[109] and alpha 2,8-linked sialic acid residues [110]. There appears no major difference in the cleavage substrate specificity in vitro for sialidases from NDV, SeV and Mumps viruses [111] suggesting that the ability to remove sialic acids from the surface of infected cells is attributed to all three members of the paramyxovirus family that demonstrate oncolytic activities. However, it is yet to be studied to what extent the ability of viruses to remove cell-surface sialic acids contributes to improvement of immunosurveillance and to the virus-specific oncolysis.
Thus, a number of mechanisms could explain oncolytic effect of Sendai virus. The oncolytic potential might depend on the individual characteristics of the strain. The virus can directly kill cancer cells replicating in them. SeV particles induce the formation of cell syncytium. The cells fused into a syncytium can no longer divide and are doomed to collective synchronous death. Furthermore, the virus causes immune-mediated killing of malignant cells, which occurring due to a strong activation of NK, as well as by potentiating the antitumor activity of cytotoxic T-cells and antigen-stimulated dendritic cells. The viral neuraminidase can provide high specific affinity for the virus to sialic acid polymer, frequently present in excess in the membranes of malignant cells. This increases the specific affinity of the virus to the malignant cells versus normal cells.